1. Field of the Invention.
The present invention relates generally to non-semiconductor electrical and electronic elements used in printed circuit board applications and particularly to an improved package and method of packaging microminiature electronic components.
2. Description of Related Technology
Dual in-line chip carrier packages (DIPs) are well known in the field of electronics. A common example of a DIP is an integrated circuit, which is typically bonded to a ceramic carrier and electrically connected to a lead frame providing opposed rows of parallel electrical leads. The integrated circuit and ceramic carrier are normally encased in a black, rectangular plastic housing from which the leads extend.
The continuing miniaturization of electrical and electronic elements and high density mounting thereof have created increasing challenges relating to electrical isolation and mechanical interconnection. In particular, substantial difficulty exists in establishing reliable and efficient connections between fine gauge (AWG 24 to AWG 50) copper wire leads associated with various electronic elements within a given DIP. One particularly useful prior art method of connecting element leads to the lead frame terminals (or interconnecting the leads of two or more electronic elements) is disclosed in U.S. Pat. No. 5,015,981, which is illustrated herein in FIG. 1. Commonly known as xe2x80x9cinterlock basexe2x80x9d technology, this method involves routing the wire lead(s) 2 to an unused lead frame slot or channel 3 located at the edge of the non-conducting base member 10, as shown in FIGS. 1 and 2. Each of these channels is designed to receive a single conductive lead frame terminal 4, which when assembled asserts an inward bias on the package thereby forcing contact between the conductive terminals 4 of the lead frame and the electronic element lead(s) 2; see FIG. 2. This method has also typically utilized a locking mechanism, such as a small tab 12 or extension on the four corner lead terminals 14, 15, 16, 17, which locks into a plastic protrusion 18 of similar dimensions using the spring tension associated with the individual lead terminals 4 of the lead frame 34. Refer again to FIG. 2.
However, while simple, this locking mechanism design suffers from three primary disabilities: (i) the relatively low amount of normal force that the package can sustain during manufacture without dislodging or deforming the locking tabs; (ii) the localization of the resistive or reaction force provided by the locking tabs on the four corners of the package; and (iii) the ability of the tabs to provide resistive force in only one direction. These limitations ultimately translate to restrictions relating to the size of the package that can be reliably assembled. For example, the use of a prior art lead frame 34 and base member 10 as described above works well for 16 pin packages, which typically have a surface area (measured across the top of the package) on the order of 0.1 square inches. When larger packages with more surface area and leads are manufactured, however, the lead frame often dislocates and separates from the base member, thereby allowing for movement and/or loss of contact of the lead frame terminals with the component leads. This reduces the reliability of the final package as a whole, and increases the cost of manufacturing, since more defective or non-conforming devices are manufactured in a given production run. This dislocation and separation is largely a result of the increased downward force associated with transfer molding the larger package. Additionally, as previously noted, the distribution of force in the larger package with respect to the leads is less desirable, since the spacing between the four locking tabs on the corners of the package is increased, thereby allowing greater flexing and distortion of the leads interposed there between. The ability of the lead frame terminals 4 to move in one direction also contributed to device failure in that the potential for misalignment and separation of the lead frame and base.
One xe2x80x9cwork-aroundxe2x80x9d solution for these problems has comprised the use of an adhesive or epoxy placed between the lead frame terminals 4 and the base member 10 so as to maintain a rigid contact between the two. However, this solution obviates many of the benefits of the interlock base technology by introducing additional process steps, materials, and curing times.
In a somewhat unrelated aspect, the aforementioned interlock base technology suffered from another disability; namely, a significant potential for shearing off of the soldered leads after formation. Specifically, one prior design of the interlock base used a series of xe2x80x9cthrough-holesxe2x80x9d 13 designed to accommodate multiple wire leads 2 and facilitate their bonding via a soldering process, as shown in FIGS. 3a and 3b. See also Applicant""s pending U.S. patent application Ser. No. 08/791,247, entitled xe2x80x9cThrough-Hole Interconnect Device With Isolated Wire Leads and Component Barriersxe2x80x9d filed Jan. 30, 1997, which is incorporated herein by reference in its entirety. Basically, the wire leads to be joined were twisted and routed through the through-hole so as to protrude from the bottom of the package as shown in FIG. 3a herein. Through-holes having an essentially flat or unchamfered bottom surface were used. During soldering, there was a tendency for the molten solder 19 to form a bubble 20 around the egress point of the leads from the feed-through hole at the bottom surface. This bubble effectively displaced solder from the leads as shown in FIG. 3b, thereby making the formation of the solder joint occur at a position lower on the leads than would occur if the bubble were not present. When the extensive portion of leads was subsequently trimmed, the entire solder joint would sometimes be inadvertently trimmed off as well, thereby potentially resulting in failure of the joint.
Based on the foregoing, it would be highly desirable to provide an improved apparatus and method for connecting a lead frame to a package of any size such that the physical forces associated with molding of the package and soldering of the leads would not result in movement or separation of the lead frame from the interlock base or wire leads disposed within the lead channels. Additionally, such an improved apparatus and method would allow for more complete soldering of any electrical joints located within feed-through holes within the interlock base, thereby reducing or eliminating failure of these joints due to inadvertent removal during processing.
The present invention satisfies the aforementioned needs by providing an improved microelectronic component package and interconnect device having a plurality of specially shaped lead channels which allow lead terminals to be more rigidly captured therein.
In a first aspect of the invention, an improved microelectronic device base member is disclosed which is fabricated from non-conductive material and includes at least one electronic component recess and a plurality of specially shaped lead channels. These lead channels are adapted to receive specially shaped lead terminals associated with a lead frame such that the lead frame and terminals are restricted from any significant movement within the lead channels during device fabrication. In one embodiment, the shapes of the lead channels and lead terminals is such so as to prevent longitudinal movement of the lead terminals within the channels in either direction, yet facilitate easy assembly. Such an arrangement allows the device to be easily assembled without additional labor or process steps, and also allows the fabrication of larger packages with a high degree of reliability. The disclosed base member also optionally includes a plurality of chamfered through-holes which permit more secure joining of the wire leads of the electronic component(s) when installed within the base member.
In a second aspect of the invention, an improved microelectronic device is disclosed utilizing the aforementioned base member and lead terminals. The device includes at least one electronic component having wire leads which is disposed within the base member, and a plurality of shaped lead terminals received within the shaped lead channels of the base member. The shape of the lead terminals and corresponding channels is such to restrict the movement of the lead terminals (and associated lead frame) within the channels during device fabrication, thereby allowing for constant and firm contact between the component wire leads disposed within the lead channels and the lead terminals. This design accordingly allows the reliable manufacturing or larger devices with a minimum of process steps. The device is also encapsulated in a polymer overmolding.
In a third aspect of the invention, an improved method for fabricating the aforementioned device is disclosed. In one embodiment of the method, the aforementioned base member is formed from a non-conductive material using a molding process. The electronic component(s) and lead frame are also formed. The electronic component is inserted in the recess of the base member, and its wire leads routed through one or more of the lead channels in the base member. Additionally, any wire leads desired to be joined with those of other components are twisted and inserted in the through-holes such that they protrude from the bottom of the base member. Next, the lead frame is mounted on the base member such that the shaped lead terminals are received and locked within the corresponding shaped lead channels, thereby forming a rugged electrical contact between the lead terminals and any wire leads routed in the channels. The base member, wire leads, and lead terminals are then dip-soldered to form permanent electrical joints. The wire leads are trimmed, and the device encapsulated in a polymer overmolding using a transfer molding process. The extensive portions of the lead terminals are then trimmed from the lead frame and formed to the desired shape.